Magnetic thin films manufacturing process



Dec. 15, 1970 f JEAN-PIERRE DUMAS ETAL 3,547,694

MAGNETIC THIN FILMS MANUFACTURING PROCESS Filed Dec. &. 19m

US. Cl. 117--240 7 Claims ABSTRACT OF THE DISCLOSURE Thin magnetic films obtained by vacuum deposition of iron-nickel alloys are made to have improved magnetic properties by causing the film to contain as an additive in very small amount a suboxide of silicon or a metal having an atomic number between 22 and 32.

BACKGROUND OF INVENTION The present invention is related to a manufacturing process of magnetic films obtained through vacuum evaporation of an iron-nickel alloy. It is Well known that the best quality films are obtained through vacuum evaporation as far as fast memories are concerned, although other processes are also used such as electrolytic deposition. However, such thin films show an objectionably high anisotropy field. Several processes have been proposed in order to reduce it. US. Patent No. 3,065,105 filed on June 12, 1958, discloses the use of a rotating magnetic field during the vacuum evaporation. This requires a quite slow evaporating rate which is objectionable as soon as production is considered. French Patent No. 1,299,152 filed on June 27, 1961, describes an anneal of the film under a rotating magnetic field. The reduction in anisotropy of the film increases with the duration of the treatment, It should at least last for minutes. This is also objectionable for production. French Patent No. 1,391,859 filed on J an. 24, 1964, describes a chemical process to decrease the anisotropy field of the film. This patent discloses the incorporation in the film of a metal additive selected among the elements with an atomic number between 22 (titanium) and 32 (germanium) or among the elements of the fourth group of the Mendeleiev classification (carbon, silicon, etc.).

BRIEF SUMMARY OF THE INVENTION n r The present invention is an improvement over the last cited French patent. According to the present invention, the iron-nickel film incorporates as an additive a suboxide or a mixture of suboxides of one or several metals mentioned in the above French patent, that is either metals the atomic number of which is comprised between 22 and 32 or metals of the fourth column of the Mendeleiev classification. The relative concentration of said additive is between 1 and 10 parts by weight for each 10,000 parts by Weight of the iron-nickel alloy.

The term suboxide as used in this specification and the accompanying claims means an oxide of an element which is an incompletely oxidized element, i.e., an oxide of the element that is unsaturated as regards the maximum oxygen contents of oxide compounds of the element.

The use of suboxides as additive improves the magnetic behaviour of thin magnetic films manufactured by vacuum evaporation under production conditions. The main advantage of the improvement lies in the fact that United States Patent O the decrease in anisotropy is obtained without increasing the angular dispersion of the easy axis of magnetisation. The decrease in anisotropy will reach as much as 30% of its original value without additive. As will easily be appreciated by the man of art, a low angular dispersion is very important to memory designers. Indeed, in a memory, the magnetic thin film is associated with two series of parallel conductors, each conductor operating on a large number of memory dots on the film. The use of suboxides as additive will simplify the manufacturing, process with respect to the use of metals. Indeed, as will be explained further in more detail, it is often possible to incorporate the suboxide or the mixture thereof to the iron-nickel ingot and thus the addition does not require a special manufacturing step. In the case of a metal additive it is often necessary to add the metal through an independent step. Moreover and more important still the evaporation rate can be substantially increased with respect to the rate without additive or with metal additive. The rate increases by a'factor 3 with respect to the process without additive to supply the same quality film.

Another advantage of the suboxide additive according to the invention lies in the favorable effect it has on the magnetic characteristics of the films with respect to memory design after magnetic annealing. The addition of suboxides according to the invention should be distinguished from the relatively thick under layer of silicon monoxide which is sometimes deposited on the thin film plate in order to improve its surface be fore vacuum deposition of the thin film. Such a procedure is described for instance in the Journal of Applied Physics by Mr. Bertelsen, page 2026, volume 33 (1962).

DETAILED DESCRIPTION OF THE PROCESS The invention will be better understood by reference to the following description and drawing which is a sketch of a vacuum apparatus to produce thin films according to the invention.

EXAMPLE 1 The suboxide which is used in this example is silicon monoxide. The monoxide powder is passed through a sieve with a mesh of .25 mm. diameter. It is added to the ironnickel carbonyl 8218% powder mixture used to manufacture the ingot which will be used as the source of thin film material.

The substrate to be coated is a. glass plate previously cleaned according to current practice in vacuum deposition technique. The plate is heated at 350 C. under a pressure of 10- torr, The ingot is evaporated through electron bombardment without a crucible. A molten drop is formed on the ingot upper part by the impinging electron beam and alloy evaporates. When an inert plate is used as in the present case, the selection of the suboxide is only based on the following condition: the evaporation temperature of the iron-nickel alloy must be quite different from the sublimation temperature of the additive (and preferably lower). In the case of silicon monoxide, solid particles of monoxide may be seen moving in the molten drop on the ingot. The electron bombardment is set so as to obtain an evaporating rate of angstroms per second on a plate located at 25 cm. from the source. As may be appreciated by the man of art this value is quite higher than current practice values, about three times as high. This is due to the addition of the suboxides in the ingot according to the invention. The silicon monoxide improves the molten surface state of the ingot. When the required film thickness between about 50 to 200 angstroms is reached, the plate is annealed under a vari- 3 able magnetic field according to current practice. The experience has shown that at this stage also the additive has a favorable influence. Prior to the anneal stage the characteristics of the film are as follows:

coercitive field: 1.5 oe. anisotropy field: 2.0 oe. angular dispersion: .5

After annealing, the characteristics are as follows:

coercitive field: 1.3 e. anisotropy field: 1.3 oe.

angular dispersion: .5

coercitive field decreases from: 2.2 oe. to 1.7 oe. anisotropy field decreases from: 2.4 oe. to 1.7 oe. angular dispersion increases from: 1.1 to 1.7"

This increase of the angular dispersion spoils the read signal from the film and limits the possibility of using such films for designing nondestructive read-out mem- 0 ories. As shown above, the films incorporating suboxides according to the teaching of the present invention to not show an increase of dispersion due to annealing. They can therefor benefit from the treatment as far as the anisotropy field is concerned. g

It is difiicult to explain clearly the influence of the monoxide in the evaporating process. It appears that the monoxide remains as solid inclusions in the ingot, but the relative number of such inclusions does not seem to remain constant during the evaporation of the ingot. It is likely that impoverishment in monoxide occurs. The characteristics of the films will not remain as satisfactory during the whole life of a given ingot as they are at the beginning of the process. More precisely the anisotropy field will keep the above value during a given number of evaporations and afterwards increases as the ingot is evaporated out. The experience has shown that it is not possible to reload the ingot with monoxide during the film production since the added monoxide powder rolls out from the molten drop of the ingot without wetting the alloy. Due to electron bombardment, electric charges buildup on the grains of the powder and they follow the electric field established in the vacuum vessel.

EXAMPLE 2 Impoverishment of the ingot in additive as encountered in the first example can be compensated for by using a mixture of two different suboxides: silicon monoxide which is incorporated in the ingot as mentioned above and titanium monoxide added to the ingot during the evaporation process. Titanium monoxide is placed in a crucible which is introduced in the vacuum evaporation apparatus and heated to a temperature below the melting point of the titanium oxide The titanium oxide is progressively poured on the molten part of the ingot during the process. Experience has shown that the addition of titanium monoxide under the operating conditions mentioned in Example 1 allows for the manufacture of high quality films until the ingot has been completely evaporated out. Titanium monoxide wets correctly the molten alloy.

EXAMPLE 3 Improvement of the film characteristics may also be obtained from suboxide formed as a thin under-coating of the plate before evaporation of the magnetic alloy film. The same characteristics as above have been obtained with an under-coating of silicon monoxide 50 to 100 angstroms thick covered with an evaporated magnetic film 200 to 500 angstroms thick. Both the undercoating and the film are obtained through vacuum evaporation. It is assumable that the monoxide diffuses into the magnetic film during the evaporation of such.

Such a very thin undercoating of suboxide is to be differentiated from the silicon monoxide layer sometimes deposited on the plate before coating with the magnetic film as mentioned for instance by Bertelsen in Journal of Applied Physics, page 2026 of volume 33 (1962). The layer which is referred to in this article is much thicker than the undercoating used in Example 3 and is intended for smoothness purposes. It does not chemically react with the metals or alloy of the magnetic film.

The drawing is a sketch of the apparatus used for production evaporation of the magnetic films according to the invention. The lower part of the vacuum vessel 1 is connected to the pumping equipment 2 not shown. Ingot 3 of the magnetic alloy is carried on support 7, water cooled as shown at the inlet and outlet of tubings 8 and 9. Plate 4 which is to receive the film is located at the upper part of the vessel. Electron gun cathode 5 supplies the electrons for melting the upper part of ingot 3. It has a frusto-conical general shape with emitting surfaces shown at 6. The electrodes necessary to focus the electron beam on the ingot surface are not represented. The electron gun assembly is described in detail in French Patent No. 1,371,219 filed on Mar. 19, 1963. Plate 4 may be heated by resistor 10. A crucible 11 which contains the oxide is located near the plate. It is supported by movable arm 12 rotatable around vertical axis 13 under a control shown as external handle 14. Of course, any type of manual or automatic control of the position of crucible 11 can be used. The crucible is represented in the position corresponding to the evaporation of the undercoating of the suboxide. During this step it occupies the axis of the vessel. 11 is a double walled crucible with the heater between the two walls. The operating temperature of the crucible is chosen such that the evaporating rate of the suboxide is about 50 angstroms per second. It is heated for two seconds. The crucible is then rotated to the position shown in dotted lines, so as to stop deposition of the oxide on plate 4. Then the magnetic alloy is evaporated according to current practice. The elapsed time between the two operating steps is advantageously shortened to the minimum. Plate 4 is heated at 350 for alloy evaporation by means of resistor 10. The evaporating rate is about angstroms per second and lasts for 3 to 6 seconds.

With a manganese suboxide mixture as the undercoating the anisotropy field of the magnetic film manufactured is comprised between 1.5 and 2 oersted. When the same process is used, without the undercoating the anistotropy field value is between 3 and 3.5 oersted, the other magnetic characteristics remaining equal for both types of films.

Of course, the sketch just described is only given for explanation purposes. In production a large number of plates are successively coated from the same ingot without breaking the vacuum. But for each plate the process is as described. Mechanical means are provided to supply a new plate 4 after each evaporation and to carry out the processed plates.

EXAMPLE 4 The undercoating mentioned in Example 3 may also be obtained through further oxidation of a metal layer. With the same apparatus as just described, crucible 11 is filled with titanium metal powder. It is then heated to a temperature higher than the evaporating point of titanium for 2 to 5 seconds. A layer of 50 angstroms is obtained after 2 seconds. The crucible is rotated off. The undercoating is left for 3 to 4 minutes in contact with the residual atmosphere of the vacuum vessel before initiating the magnetic film evaporation step. The characteristics of the films are very similar to those obtained in the third example.

When other metals are used, it may be necessary to introduce an oxidizing atmosphere in the vacuum vessel after evaporation of the undercoating. Of course, such an atmosphere is to be evacuated before proceeding with the evaporation of the magnetic alloy.

We claim:

1. A magnetic thin film manufacturing process comprising the following steps:

providing a magnetic alloy ingot from nickel and iron incorporating from 0.001 and 0.0001 part by weight of silicon monoxide per part of nickel-iron alloy;

preparing a batch of plates;

introducing said ingot and batch in a vacuum vessel;

establishing in said vessel a vacuum of about torr;

heating the first plate of said batch;

heating said ingot to evaporate a film of said alloy on said plate about 50 to 200 angstroms thick;

removing said first plate;

heating the second plate of said batch;

exposing said plate to said alloy vapor stream and so on until the ingot is completely evaporated out.

2. A magnetic thin film manufacturing process comprising in combination the following step:

(a) providing a magnetic alloy ingot of nickel and iron containing between 1 and 10 parts by weight per 10,000 parts of the alloy of suboxide of metal selected from the group consisting of silicon and metals having an atomic number from 22 to 32,

(b) providing a batch of plates,

(c) introducing said ingot and said plates into a vacuum vessel,

((1) establishing in the vessel a vacuum of about 10 torr,

(e) heating the first of said plates,

tOIT,

(f) heating said ingot to evaporate onto said heated plate a film 50 to 200 angstroms thick,

(g) removing said first plate to a store,

(h) heating the second of said plates,

(i) exposing said heated second plate to alloy vapors evaporated from said ingot, and

(j) repeating the removing of said plates and said exposing to evaporate onto each of the plates of the batch of film 50 to 200 angstroms thick.

3. A process as claimed in claim 2 wherein said suboxide is titanium monoxide.

4. A magnetic thin film manufacturing process comprising in combination the following steps:

' (a) providing a magnetic alloy ingot of nickel and iron containing between 1 and 10 parts by weight per 10,000 parts by weight of the alloy of silicon monoxide,

(b) providing a batch of plates,

(c) filling a crucible with a suboxide of titanium,

(cl) introducing said ingot, said plates and said crucible into a vacuum vessel,

(e) establishing in said vessel a vacuum of about 10 torr,

(f) heating the first of said plates and placing it adjacent said ingot,

(g) heating said ingot to evaporate on said first plate a film of said alloy 50 to 200 angstroms thick,

(h) removing said first plate to a store,

(i) performing the same operation on a plurality of said plates,

(j) heating said crucible to a temperature below the melting point of the enclosed suboxide,

(k) pouring a portion of said heated suboxide from the crucible onto the molten surface of said ingot,

(l) repeating the steps (f), (g) and (h) on a further plurality of said plates,

(in) repeating the steps (j) and (k), and

(n) continuing said repetitive operations until said ingot has been completely evaporated.

5. A magnetic thin film process comprising in combination the following steps:

(a) providing an ingot of magnetic alloy of iron and nickel,

(b) providing a source of suboxide of metal selected from the group consisting of silicon, titanium and manganese,

(0) providing a batch of plates,

(d) introducing said ingot, said source and said plates into a vacuum vessel,

(e) establishing in said vessel a vacuum of about 10- torr,

(f) placing the first plate of said batch adjacent said source,

(g) heating said source to vaporize onto said first plate a film of the suboxide of said source about 50 to 100 angstroms thick,

(h) removing said first plate from said source,

(i) heating said first plate,

(j) placing said heated first plate adjacent said ingot,

(k) heating said ingot to evaporate onto said first plate a film of the alloy of said ingot about 200 to 500 angstroms thick,

(1) removing said first plate to a store, and

(In) repeating steps (f) through (1) upon succeeding plates of said batch to form upon each of said plates a film of suboxide of said source about 50 to 100 angstroms thick overlayed with a film of said alloy about 200 to 500 angstroms thick.

6. A process as claimed in claim 5 wherein said suboxide is silicon monoxide.

'7. A magnetic thin film manufacturing process comprising in combination the following steps:

(a) providing an ingot of magnetic alloy of iron and nickel,

(b) providing a source of metal selected from the group consisting of silicon, titanium and manganese,

(c) providing a batch of plates,

(d) introducing said ingot, said source and said batch of plates into a vacuum vessel,

(e) establishing in said vessel a vacuum of about 10- torr,

(f) placing the first plate of said batch adjacent said source,

(g) heating said source to evaporate on said plate a film of said group metal of thickness about 50 angstroms,

(h) removing said plate from adjacent said source,

(i) leaving said first plate inside said vacuum vessel to oxidize said film on said plate by action of oxygen contained in the atmosphere within said vessel,

(j) placing said first plate adjacent said ingot,

(k) heating said ingot to vaporize onto said first plate a film of said alloy about 50 to 200 angstroms thick,

(1) removing said first plate to a store, and

(m) repeating said steps (f) through (1) upon further plates of said batch.

References Cited UNITED STATES PATENTS 2,900,282 8/1959 Rubens 11723 8X 3,065,105 11/1962 Pohm 117-107X 3,093,507 6/1963 Lander et :al. 117106 FOREIGN PATENTS 1,299,152 6/1962 France.

OTHER REFERENCES Bertelsen, Journal of Applied Physics, vol. 33, No. 6,

ALFRED L. LEAVITT, Primary Examiner W. E. BALL, Assistant Examiner US. Cl. X.R. 

